6-aminocaproic acid of formula (I) is commonly used for the preparation of plastic materials, but it is also used as medicine for the treatment of coagulopathies and severe post-operative bleedings. The drug is currently commercialized by Clover Pharmaceuticals Corp. with the brand name Amicar®, both as solutions for oral use (0.25 g/mL) and as tablets (500 mg or 1000 mg).

6-aminocaproic acid of formula (I) used in the polymer industry makes it a compound readily available at a low cost, but the product necessary for medical use has not the quality to be suitable as active pharmaceutical ingredient.
The synthesis of 6-aminocaproic acid is known for a while and is generally carried out by hydrolyzing caprolactam of formula (II).

The reaction can be carried out under acidic or basic conditions and there are no major reactivity issues. At the end of the hydrolysis a solution of a 6-aminocaproic acid salt is obtained and the desired product is isolated by neutralizing the mixture, which provides the free amino acid.
However, the isolation of the 6-aminocaproic acid of formula (I) as such is rather challenging, because 6-aminocaproic acid is a very polar molecule with a high solubility in water. Due to these properties the liquid/liquid extraction with an organic solvent after the hydrolysis and neutralization results in the best situations to a partial extraction into the organic phase, but more commonly the product remains totally in the aqueous phase. By concentrating the neutralized solution it is possible to isolate the 6-aminocaproic acid by precipitation, but the precipitate is contaminated with inorganic salts formed during the neutralization.
Different approaches have been disclosed for obtaining a product with a greater purity. For example, Meyers and Miller describe in Organic Syntheses, Coll. Vol. 4, p. 39 (1963) that the 6-aminocaproic acid hydrochloride salt of formula (Y)
can be released by ion exchange chromatography. However, this process is not suitable for industrial applications, because it is well known that ion exchange chromatography is particularly expensive and labor intensive due to the use of the resins.
In Organic Syntheses, Coll. Vol. 2, p. 28 (1943) the hydrochloride salt of 6-aminocaproic acid of formula (Y) is converted into the 6-aminocaproic acid of formula (I) by treating the aqueous solution with lead oxide, lead hydroxide, and silver oxide, and finally sulfuric acid in order to remove all metal ions and halogenates.
U.S. Pat. No. 3,655,748 discloses that the hydrolysis of caprolactam of formula (II) is carried out in presence of barium hydroxide. At the end of the reaction the solution is neutralized by adding CO2, and the formed barium carbonate precipitate is filtered off and removed quantitatively from the solution. The aqueous solution, which comprises only 6-aminocaproic acid of formula (I), is then concentrated providing 6-aminocaproic acid of formula (I) as solid.
The method described in U.S. Pat. No. 8,809,581 seeks to solve the above cited issue of the workup by treating the reaction mixture of the hydrolyzed caprolactam of formula (II) with a reagent able to introduce a protecting group, which can be removed afterwards by hydrogenation. By this way, the protected 6-aminocaproic acid has a good solubility in organic solvents, thus can be extracted into the organic phase and then finally deprotected by hydrogenation. However, this procedure comprises a further synthetic step in the production process and the use of palladium based catalysts in the last step of the synthesis. In this process it is necessary to completely remove the catalyst in order to guarantee the absence of heavy metals in the final product.
Thus, there is still the need to have a more suitable alternative method for isolating and purifying 6-aminocaproic acid of formula (I).
The inventors of the present invention have surprisingly found that the purification of 6-aminocaproic acid of formula (I) can be performed effectively, also at an industrial scale, by removing the addition salt of 6-aminocaproic acid of formula (Ia), as herein defined, by an organic base in a solvent. The new method results to be advantageous in respect to the procedures known in the art, since it is industrially scalable, it does not comprise the use or the formation of toxic compounds, nor the use of ion exchange resins or heavy metals, and the 6-aminocaproic acid of formula (I) obtained by this procedure contains a minimal amount of impurities, in particular the one of formula (III) as described below.